Continue then with the cap itself. Again, a few pics while disassembling the cap will help when it's time to reassemble.
Continue then with the cap itself. Again, a few pics while disassembling the cap will help when it's time to reassemble.


Note the non-alignment of the two vent holes, one in the metal plate, and the other in the fiber washer. Note also that the screw holes fit in one and only one way on reassembly, and that the vent hole in the side of the fiber washer facing the camera will also be offset from the hole in the larger metal plate on reassembly. This is to allow venting without splashing gas out through the cap.
 

These metal parts I soaked in Berryman's carb cleaner, worked over with a brass brush, and then washed with soap and water. Except for the chromed cap itself.
 
These parts I cleaned with WD-40.
 
This plastic part I washed with soap and water.
 
And these are the new and/or parts not needing cleaned. This cap is really not in bad shape at all.
 

Reassembly is pretty straight forward until a point.
 
 
 
Then comes the snag. It seems like there should be a way to assemble the large rubber piece onto the cap and proceed from there. And if I am missing something, please jump in with a post and correct me. But I have tried putting the rubber ring in and then lifting it onto the large metal plate much like mounting a tire, but I have met with no success that way.
  
And if you try to assemble the two metal plates with the rubber ring and then attach the whole assembly to the underside of the chromed cap...the large metal plate completely blocks access to the center screw pictured in the third pic of post #22.

The way I see to do it is, after slipping the rubber gasket in place on the large plate (not shown), to insert the three screws through the large metal plate and into the fiber washer just enough to hold the washer in place, and then put the large metal plate in place on top of the smaller plate, working the smaller opening in the rubber ring over the smaller plate and the large spring. It's hard to photograph.
  
You are working a bit upside down to do this. Again, if someone has a better way to do it, let us know. I wondered if the rubber ring doesn't go in reverse, with the smaller hole out, but both caps I have disassembled were put together this way, and the three-piece caps match this assembly with the way the large rubber ring's large opening seals the tank opening.

Reassemble the locking system according to your photos, and replace it onto the tank with the screw. 
 
 

Thanks for the detailed instructions.  I wonder if you could use some string to pull that seal lip over.  My idea is basically the same trick used to install windshields on old cars.  To do this, you stuff some string along the inside of the lip of the seal. Basically, run the string along the inside of the lip and leave a bit of overlap on the string.  Leave some string hanging out. Just enough to pull on. Once everything is assembled, apply pressure to the seal against the plate just ahead of the string you are going to pull on. You have to pinch the string inside the seal or it will just slip out to the middle (maybe easier said than done). Slowly pull the string and inch along the perimeter of the seal.  The string should pull the lip out and over the plate as you draw it out.  

I hope this is helpful.  Apologies if i misunderstood the task here but figured its better to throw it out there and see if it helps. Cheers!

 Okay. Forget my last post. I had some time to disassemble the gas cap from my DT360 and figured out how it goes together. I have attached some pictures to help show what I am talking about. Basically there is a spring clip that goes under the centre screw. In your photo its installed upside down (Post 22). When installed properly it allows you to preassemble everything and simply press it on.   Okay. Forget my last post. I had some time to disassemble the gas cap from my DT360 and figured out how it goes together. I have attached some pictures to help show what I am talking about. Basically there is a spring clip that goes under the centre screen. In your photo its installed upside down (Post 22). When installed properly it allows you to preassemble everything and simply press it on. 

Brilliant! Thanks for the correction. I remembering wondering about that piece when I first took it apart! I am a victim of both the PO of not one but two bikes, as it was installed this way in both a 1974 DT175A and a 1974 DT250A. But that explains it perfectly! I had a hard time believing the Japanese engineers would have required such a devilish reassembly process. Maybe engineers of other nations closer to home(hint, hint), but not the Japanese.
I should have flipped it just for kicks, to rule out that way. But I trusted the PO's.

Thanks again!

Happy to be of help.  This site has helped me with so many things so it’s nice to be able to help someone.

Cheers

Does anyone have a detailed expanded view of the Gas Tank Lid Assembly for a 1973 RT3?
Thanks